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QM says that there is no such thing as a quantity's definite value. You describe an object probabilistically before measuring the value. This description is probabilistic not because you don't know the spin. It is inherently probabilistic. That is how Nature is. Even the electron doesn't know what its spin is!Of course, on measuring the value, you get one of its allowed values. \(+\frac{1}{2}\) or \(-\frac{1}{2}\) for the spin; 1 unit or 1.5 units for the distance, etc. But, until you measure, it is just probability.